LAUSR

Actinobacillus succinogenes is a natural succinate producer, which is the result of fumarate respiration. Succinate production from anaerobic growth with C4 -dicarboxylates requires transporters catalysing uptake and efflux of C4 -dicarboxylates. Transporter Asuc_1999 (DcuE) found in A. succinogenes belongs to the Dcu family and was considered the main transporter for fumarate respiration. However, deletion of dcuE affected l-malate uptake of A. succinogenes rather than fumarate uptake. DcuE complemented anaerobic growth of Escherichia coli on l-malate or fumarate; thus, the transporter was characterized in E. coli heterologously. Time-dependent uptake and competitive inhibition assays demonstrated that l-malate is the most preferred substrate for uptake by DcuE. The Vmax of DcuE for l-malate was 20.04 μmol/gDW·min with Km of 57 μM. The Vmax for l-malate was comparable to that for fumarate, whereas the Km for l-malate was 8 times lower than that for fumarate. The catalytic efficiency of DcuE for l-malate was 7.3-fold higher than that for fumarate, showing high efficiency and high affinity for l-malate. Furthermore, DcuE catalysed the reversible exchange of three C4 -dicarboxylates - l-malate, fumarate and succinate - but the preferred substrate for uptake was l-malate. Under physiological conditions, the C4 -dicarboxylates were reduced to succinate. Therefore, DcuE is proposed as the l-malate/succinate antiporter in A. succinogenes.